The present invention concerns a final, ready to use, spacer grid configured to separate and hold nuclear fuel rods in a nuclear reactor of the boiling water reactor (BWR) type in predetermined positions relative to each other.
A nuclear boiling water reactor comprises a core having a plurality of fuel assemblies. Each fuel assembly includes a plurality of fuel rods and each fuel rod comprises nuclear fuel enclosed by a cladding. The fuel rods are held in predetermined positions relative one another with the help of a number of axially distributed spacer grids, each spacer grid consisting of a lattice structure with a number of cells through which the fuel rods extend.
The environment in the core of a nuclear BWR is demanding for the components positioned therein. The environment is highly oxidative. A spacer grid must for example withstand the following circumstances: a two phase flow of steam and water at a temperature of about 286° C., wherein the flow of the steam is 10 m/s and the pressure is 70 bar. There are water droplets in the steam and an oxygen content and hydrogen peroxide content in the environment of 0.4 ppm and <1 ppm, respectively. The spacer grid is also exposed to strong radiation.
Spacer grids are often produced from thin metallic plates of zirconium alloys or Ni base alloys. A well-known Ni base alloy is called X-750. Alloy X-750 has been used for BWR spacer grids with considerable success for more than 30 years. A drawback with spacer grids made from alloy X-750 is however that a relatively high corrosion rate in some reactors, due to the specific environment described above, results in general corrosion of the spacer grid surface. The general corrosion of the spacer grid may lead to a release of 58Co into the reactor water. 58Co is an isotope of Co and it deposits onto surfaces in the nuclear reactor. 58Co is mainly formed through neutron activation of 58Ni. Furthermore, another isotope of Co, 60Co, is formed by neutron activation of the common isotope 59Co. Both 58Co and 60Co are radioactive isotopes and the release of these radioactive isotopes into the reactor water results in an increased risk for exposure of staff working at the nuclear reactor plant.
The term “Ni base alloy” does in this context mean that the principal element in the alloy is Ni. No other element is present in a greater amount. A Ni base alloy has a matrix made up of Ni with other elements such as Cr and Fe in solution. By heat treatment of the alloy so called γ′ secondary phase particles may be formed by changes in solid solubility with temperature. The fine γ′ secondary phase particles prevent the movement of dislocations, or defects, in the matrix of the alloy, thereby increasing the mechanical strength of the material. The γ′ secondary phase particles in a Ni base alloy are normally Ni3(Ti,Al).
JP 09-324233 A describes a Ni base alloy of high strength stated to have an improved resistance against stress corrosion cracking (SCC). The alloy is especially intended to be used in components such as springs, bolts and pins inside the high temperature hot water environment of a nuclear BWR or a nuclear PWR (pressurized water reactor). The alloy is similar to alloy X-750. JP 09-324233 A does however state a higher amount of Fe compared to alloy X-750. By increasing the amount of Fe, JP 09-324233 A states that an improved SCC resistance is obtained.
Stress corrosion cracking of a metal occurs due to a constant tensile stress of the metal in a corrosive environment, especially at elevated temperatures. Stress corrosion usually leaves most of the surface of a component unattacked, but takes place at the positions that are exposed to the constant tensile stress. Fine cracks are formed in the material and the cracking may lead to an unexpected sudden failure of the metal. Springs, bolts and pins inside the high temperature hot water environment of a nuclear BWR or a nuclear PWR are, as described in JP 09-324233 A, examples of components that are subjected to SCC.
In the core of a nuclear BWR general corrosion may occur, which may result in an undesired release of radioactive Co isotopes as described above. General corrosion is a particular problem in the core of the reactor due to the particular conditions that exist there. General corrosion can take place all over the surface of a component, the corrosion being characterised by a uniform attack. Since a spacer grid is positioned inside the core of the reactor, it is particularly subjected to general corrosion.
As mentioned above, a drawback with spacer grids made from alloy X-750 is that the relatively high corrosion rate in some reactors leads to general corrosion on the surface of the spacer grid. There is therefore a desire to improve the corrosion resistance of the spacer grid.